Signal processing system for locating a transmitter

ABSTRACT

A signal processing system is provided which describes a method and apparatus to determine the absolute time of arrival of a transmitted signal and to determine the location of a transmitting device by determining the relative difference in time that a transmitted signal is received at three or more receivers which are spaced apart with respect to each other. The method and apparatus is applicable to all systems which transmit discrete signals or signals which comprise composite components. The system comprises a ramp detection component to track the arrival of the signal and to determine, to a high time resolution, the arrival of a signal by identifying a particular feature of the transmitted signal. The time-of-arrival of the signal is determined with reference to a timing circuit. Each receiver further comprises a timing-synchronisation component to maintain a highly accurate clock by using an accurate timing reference such as the timing signals transmitted by the Global Positioning System constellation of satellites. In one embodiment the method is applicable to the transmission of discrete signals according to the protocol of a packet switching network.

BACKGROUND OF THE INVENTION

Signal processing methods which determine the time-of-arrival of signalsare well known. It is known to read frequency modulated coded signalsand to determine a time of arrival of these signals to the timeresolution given by the inverse of the bandwidth of the signal. To date,no signal processing methods and apparatuses have been described whichcan improve upon this resolution by analysing the signal in terms of itscomposite components.

Packet switching networks are known such as modacom and Mobitex. Thesenetworks are exclusively operated as messaging networks and they operateaccording to a structured protocol in which remote units requesttransmission windows to communicate with the nearest processing basestation. The transmitted data packets are of very short duration andeach comprises identification code information identifying the unit fromwhich the message was sent. To date, no methods have been proposed toanalyse the signals transmitted by remote units operating in accordancewith said packet switching networks and thus provide a positioningcapability to the packet switching network's messaging capability.

Conventional networks determine the approximate location of GSMtelephones and personal message pagers to the resolution of a cell basedupon the signal strength of their transmissions. Communication channelsare allocated to that base station which receives the strongest signalfrom the transmitting unit. Conventional networks do not process signalsto determine the location of transmitting units to a high resolution.

Furthermore, no signal processing system is presently available whichserves to analyse a discrete signal in terms of its composite componentsand thereby greatly improve upon the bandwidth limitation on thedetermination of the time-of-arrival of the signal whereby a codedsignal can be determined to a time resolution given by the inverse ofthe bandwidth. Moreover, no signal processing system of this type isknown which also serves to determine the position of a remotetransmitting unit by differential comparison of the time-of-arrival ofthe signal at a plurality of receivers in such a simple and effectivemanner as herein proposed

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a novelsignal processing system which serves to provide a highly accuratemeasure of the time of arrival of a discrete coded signal comprisingcomposite component attributes.

It is a further object of the present invention to provide such a signalprocessing system method and apparatus which is both efficient andeffective in its processing algorithm.

It is a further object of the present invention to provide the means tomake use of a differential time measurement technique which isapplicable for positioning purposes by gathering information from aplurality of receivers that analyse either a discrete coded signal suchas that transmitted according to the protocol of a packet switchingnetwork, wherein said signal comprises component parts, or acontinuously transmitted coded signal comprising composite componentparts.

It is a further object of the present invention to provide asignal-processing system comprising a time-measurement technique that isapplicable to the transmission protocol of a packet switching networksuch as that used by the paged area network as described in U.S. Pat.No. 5,051,741 (Wesby).

BRIEF SUMMARY OF THE INVENTION

Certain of the foregoing and related objects are readily-attainedaccording to the present invention by the provision of a signalprocessing system which includes the means to receive a discretelytransmitted coded signal at one or more receiving means such as thattransmitted is according to the protocol of a packet switching networkwherein the signal is composed of composite parts which may comprise afrequency modulated signal ramp of increasing amplitude from zero to amaximum followed by a frequency modulated data stream of digital bits.

1. The invention of the signal processing system relates to a method andapparatus for determining the time of arrival of a coded signal.

2. More particularly, it relates to a signal processing method andapparatus to determine the arrival of a discrete coded signal comprisingdiffering composite components such as signals used for transmittingdata over a data bus operating according to the protocol of a packetswitching network.

3. Furthermore, the invention relates to a signal processing method andapparatus for identifying discrete components of a transmitted codedsignal for determining the time-of-arrival of this coded signal.

4. Furthermore, the system relates to a method for comparing thedifference in time of arrival of a signal at a number of receivingstations wherein the signal may comprise a discrete coded signal such asthat used in packet switching networks or a continuously transmittedsignal comprising differing component parts which can be readilyidentifiable by the receiving means.

Alternatively the signal may comprise a continuously transmitted signalwhich includes a train of composite signal components such that thearrival of the is coded signal can be time-determined relative to one ofthese components.

Alternatively and including the above examples, the signal may betransmitted using one or more of any transmission mediums including anypossible encoding.

In all cases, the receiver divides the signal into its component partsand recognises the components of the signal. In the particular case of adiscrete coded signal transmitted according to the protocol of a packetswitching network, the receiver tracks the arrival of the signal as ittraces a frequency-modulated bandwidth-limited ramp, increasing inamplitude up to the maximum operational amplitude, which is frequencymodulated according to known data modulation principles with a specifiedbandwidth about a carrier frequency.

Furthermore, the receiver identifies the rate of change of the amplitudeof the signal as it increases up to a maximum amplitude.

Furthermore, the receiver stores the transmitted coded signal andanalyses it subsequent to storage. Furthermore the receiver provides ahighly accurate timing reference such that the time of arrival of thetransmitted coded signal may be determined to a very hightime-resolution.

A master station communicates with the network of receiving stations todetermine which of the receiving stations has determined atime-of-arrival of a coded signal transmitted from a remote transmitter.

The invention relates to using the analogue part of a signal containingcoded information, whatever the form, in order to determine thetime-of-arrival of this signal, regardless of the carrier frequency, thecoding of the signal (PSK, FSK, etc.) and regardless of the datacontained in the signal. The method according to the invention isapplicable to all signals whose characteristics are known.

Furthermore, the master station communicates periodically with each ofthe plurality of receiving stations to determine the exact difference intime between the master station's own timing means and each receiver'stiming means in order that the master station is able to combinetime-of-arrival information from different receivers wherein eachreceiver has its own unique time reference.

Furthermore, each received coded signal, transmitted from a remotetransmitter, whose time-of-arrival is determined by one of each of aplurality of receivers spaced throughout an area from within which saidsignal is transmitted, is processed by a master station which receivessaid encoded time-of-arrival information from each of the plurality ofreceivers which received the said coded transmission and combines saidinformation differentially, knowing the relative positions of each ofthe receivers, and knowing the difference in time between the masterstation's own timing means and each of the receiver's timing means suchthat the master station may determine a position from which thetransmission was sent relative to the plurality of receivers thatreceived said coded signal.

Said receivers are spaced throughout a coverage area. Each of thereceivers may be fixed or in motion. Each receiver which determines thetime of arrival and identification code by separating it from otherinformation of a signal transmission, passes this information to themaster station when directed to do so. In the instance that thereceivers are fixed, the master station knows the relative separation ofeach of the plurality of receivers spread throughout said coverage areaand the master station's own position relative to each of said pluralityof receivers. In the instance that the receivers are in motion, eitherrelative to each other or to the master station, or relative to botheach other and the master station, means exist at the master station todetermine the instantaneous relative separation of the receivers withrespect to each other and their individual relative separation withrespect to the master station at the instant that the coded transmittedsignal is received at each of the receivers that received said codedsignal.

The master station maintains an accurate timing reference such as thatprovided by the GPS (Global Positioning System) constellation ofsatellites, an atomic clock or other such means. Furthermore, the masterstation determines and updates the relative difference between themaster station's own timing reference, and that as determined by each ofthe plurality of receivers such that the master station is able tocombine timing information from different receivers differentially andthereby determine the position from which a coded signal is sent.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in connection withthe accompanying drawings which disclose one embodiment of theinvention. It is to be understood, however, that the drawings aredesigned for the purpose of illustration only and not as a definition ofthe limits of the invention and the invention is not limited to thisexemplary embodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a figurative schematic of the component parts of a receiverused in association with the method and apparatus of the presentinvention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now in detail to the drawing denoted FIG. 1, thereinillustrated is a figurative schematic of the component parts of areceiver used in association with the method and apparatus of thepresent invention. The schematic is to be interpreted in the widestsense such that each component part of the schematic comprises a powersupply where appropriate and error-correcting circuitry which mayfurther comprise data processing units and memory. Furthermore, theantennas shown may be replaced by fewer antennas where appropriate.Duplication of components is provided to make it easier to understandthe schematic.

With reference to the operation of the receiver, the transmitted signalthat is received by the receiver comprises a ramp modulated at thecarrier frequency, transmitted over a time window such that theamplitude of the signal increases up to a maximum threshold such thatusers of adjacent frequency bands do not experience interference. Thisramp signal is followed by a digital data stream, frequency-modulatedabout the carrier frequency with a bandwidth B. This digital data streamcomprises a coded signal such as an identification code of thetransmitting device.

The schematic comprises four main signal-processing-components:timing-synchronisation; ramp-detection; signal-decoding; andcommunication with the master station.

In the preferred embodiment, the timing-synchronisationsignal-processing-component comprises a global positioning system unit10 to control the receiver's own internal timing means 11 therebyproviding the receiver with the ability to maintain a highly accuratetiming means at a high timing resolution. Furthermore, the timingsynchronisation component comprises the means 12, 13, 14 to output aninstantaneous clock value for transmission to the master station suchthat the master station can determine the difference in time between themaster station's own timing means and the timing means of each receiver.

In the preferred embodiment, the ramp-detectionsignal-processing-component comprises an antenna 20, which passessignals to an envelope detector 21 which outputs signals to adifferentiator 22 such that the output is a measure of the rate ofchange in amplitude of the ramp signal. The output of the differentiator22 is subsequently passed to a threshold detector 23 which is designedto identify a particular threshold condition such that when thiscondition is identified, the threshold detector 23 then passes a signalto a trigger circuit 24 which then reads the instantaneous value of thetiming circuit 11 and passes this to a signal processing circuit 40.

In the preferred embodiment, the signal-decodingsignal-processing-component comprises an antenna 20, which passessignals to a decoder 30 for transmission to a digital data reader andcompiler 31 in which signals are analysed and compared with particularsought transmissions is and subsequently complied into a format to betransmitted to signal processing unit 40.

In the preferred embodiment the means to communicate with the masterstation comprises a signal processing unit 40 a modem 41, and an antenna42, whereby time-of-arrival signals are compiled with identificationsignals and encoded before transmission to the master station.

The receiver determines the threshold condition based upon the designparameters of the threshold detection circuits. The threshold conditionis determined from the relative change in amplitude of the signal andnot from absolute amplitude. Although the ramping signal is subject tonoise and may be slightly different from unit to unit, the same signalis picked up at three or more receivers and each of the receivers isdesigned to trigger a threshold condition based upon the same designparameters. In this way the trigger-threshold time resolution becomesdependent upon the design parameters of the receiving circuitry and notdirectly upon the bandwidth of the transmitted signal. Noise will affectthe resolution if the signal to noise ratio is low around the triggerthreshold but this will be the same for each receiver. The noise will bedisplaced in time but it will not be of such short duration that it willaffect the threshold condition of each receiver differentially.

It is necessary to adapt the threshold detector for local noiseconditions. This is done by mathematically analysing the power of thenoise. Equations for the noise power are derived in which noise ismodelled as a normal variable. Based upon these equations the thresholdlevel is determined so that the circuit operates acceptably, in thegiven noise environment. The analytical result may be used to designtime threshold detectors for other environments.

Although this signal processing system has been described with regard toa time of arrival determination method making use of the differentialamplitude of the signal, it is also possible to achieve the same resultby analysing the phase characteristic of the signal in which case theenvelope detector 21, the differentiator 22, and the threshold detectorcircuitry are replaced with circuitry designed to analyse the phasecharacteristic.

Thus the position of the transmitting unit is determined bydifferentially comparing the trigger threshold times as determined by aplurality of receivers.

The master station measures and periodically updates the difference intime between its own accurate internal clock and the accurate clock ofeach of the receivers. When the master station receives a timing signalas measured by one of the receivers, it corrects this time by thedifference between its own clock and the clock of the particularreceiver to determine an absolute measure of the time of the thresholdcondition as measured by that particular receiver. The master stationdetermines the difference in threshold condition time as measured bythree or more receivers for the same transmitted signal from a mobiletransmitting unit to determine a position for said mobile transmittingunit.

Although the signal processing system has been described with referenceto the specific application of measuring the times of arrival ofdiscrete coded signals as transmitted according to the protocol of apacket switching network, it will be clear to anyone skilled in thedesign of such signal processing systems that it can be designed inrespect of any signal transmission system wherein each transmittedsignal comprises encoded composite components any of which may be usedto determine the relative distances of each of the receiving receiversto the place of transmission.

While only one embodiment of the present invention has been shown anddescribed, it will be obvious to those persons of ordinary skill in theart, that many changes and modifications may be made thereunto, withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A network of communication stations eachcomprising a receiver and a transmitter, spaced throughout a paged area,communicating with a master station to determine the position of aremote moveable transmitting unit, comprising at each communicationstation, a signal processing system for determining the time of arrivalof a discretely transmitted coded signal from within said paged area,composed of composite parts, said signal processing system characterisedin that it comprises: a ramp-detection means for tracing afrequency-modulated bandwidth-limited ramp further comprising; anantenna (20) which passes frequency-modulated signals to an envelopedetector (21) for detecting a bandwidth-limited ramp, increasing inamplitude up to the maximum operational amplitude which outputs signalsto a differentiator (22) which measures the rate of change in theamplitude of the ramp signal and the output of which is passed to athreshold detector (23) designed to identify a particular thresholdcondition such that when this condition is identified, the thresholddetector (23) then passes a signal to a trigger circuit (24) which thenreads the instantaneous value of internal timing means (11) and passessaid instantaneous value to a signal processing circuit (40), and atiming-synchronisation signal-processing means comprising an accuratetiming means to control the receiver's own internal timing means (11)thereby providing the receiver with the ability to maintain a highlyaccurate timing means (11) at a high timing resolution, which outputs aninstantaneous clock value to said trigger circuit (24) for transmissionto the master station such that the master station can determine thedifference in time between the master station's own timing means and thetiming means of each communication station.
 2. The network ofcommunicating stations according to claim 1 wherein said network iseither a packet switching network or a mobile communicationinfrastructure such as GSM.
 3. The network of communication stationsaccording to claim 1 wherein the signal processing system further usesthe analogue part of a demodulated received signal containing codedinformation, in order to determine the time-of-arrival of this signal,regardless of the carrier frequency, the coding of the signal andregardless of the data contained in the signal.
 4. The network ofcommunicating stations according to claim 1 further characterised inthat the master station determines the difference in threshold conditiontime as measured by three or more receivers each at a differentcommunication station, for the same transmitted signal from a mobiletransmitting unit to determine the position of said mobile transmittingunit.
 5. The network of communication stations according to claim 4wherein the master station communicates periodically with each of theplurality of receiving stations to determine the exact difference intime between the master station's own timing means and each of three ormore receiver's timing means (11) corresponding to a remote transmittingunit, in order that the master station is able to combinetime-of-arrival information from said different receivers wherein eachof said receivers has its own unique time reference.
 6. The network ofcommunication stations according to claim 5 wherein at least one of saidreceivers are in motion.
 7. The network of communication stationsaccording to claim 6 wherein said master station comprises means fordetermining the instantaneous relative separation of said receiver inmotion and said three or more receivers each at a fixed communicationstation, with respect to each other and their individual relativeseparation with respect to the master station at the instant that thecoded transmitted signal is received at each of the receiver of saidcommunication stations that received said coded signal.
 8. The networkof communication stations according to claim 1 wherein a plurality ofmoveable units comprise means for communicating with said three or morereceivers each at one of said communication station in order to provokethe transmission of the calculated time of arrival and the relativeposition of each of said receivers at each communication station, andmeans for further locally calculating its own position.
 9. The networkof communication stations according to claim 8 wherein said plurality ofmoveable units transmit their position to the master station via thecommunication stations.
 10. A method for determining the time of arrivalof a discretely transmitted coded signal composed of composite partsfrom within a network of communication stations each comprising areceiver and a transmitter, spaced throughout a paged area,communicating with a master station to determine the position of aremote moveable transmitting unit, comprising the steps of: receiving atransmitted signal sent by a moveable unit in response to a pagingsignal, the transmitted signal being received by at least threereceivers, each located in one communication station, tracing afrequency-modulated bandwidth-limited ramp by a ramp-detection means,passing the frequency-modulated signals to an envelope detector (21) fordetecting a bandwidth-limited ramp, increasing in amplitude up to themaximum operational amplitude, measuring the rate of change in theamplitude of the ramp signal by a differentiator (22), the output ofwhich is passed to a threshold detector (23), identifying a particularthreshold condition by said threshold detector (23), such that when thiscondition is identified, the threshold detector (23) passes a signal toa trigger circuit (24), reading the instantaneous value of internaltiming means (11) and passing said instantaneous value to a signalprocessing circuit (40).
 11. The method for determining the time ofarrival of a discretely transmitted coded signal composed of compositeparts according to claim 10 wherein said step of passing saidinstantaneous value to a signal processing circuit (40) comprises thesteps of: controlling the receiver's own internal timing means (11) byan accurate timing means such as a global positioning system unit (10),thereby providing the receiver with the ability to maintain a highlyaccurate timing means (11) at a high timing resolution, providing aninstantaneous clock value from said highly accurate timing means (11) toa trigger circuit (24) for transmission to the master station, andpassing said instantaneous clock value to a signal processing circuit(40).
 12. The method for determining the time of arrival of a discretelytransmitted coded signal composed of composite parts according to claim10 further comprising the steps of: combining the time of arrival anddata processed by each of said communication stations in order totransmit this information to said master station by the processing unit(40), transmitting said information to said master station from each ofsaid receivers of each of said communication station.
 13. The method fordetermining the time of arrival of a discretely transmitted coded signalcomposed of composite parts according to claim 12 further comprising thesteps of: determining the difference in threshold condition time asmeasured by three or more receivers each at a different communicationstation, for the same transmitted signal from a mobile transmitting unitby said master station, determining the position of said mobiletransmitting unit by said master station.
 14. The method for determiningthe time of arrival of a discretely transmitted coded signal composed ofcomposite parts according to claim 13 further comprising the steps of:periodically communicating with each of said plurality of saidcommunication stations, determining the exact difference in time betweensaid master station's own timing means and each receiver's timing means(11), storing the update of relative timing differences for each one ofsaid communication station.
 15. The method for determining the time ofarrival of a discretely transmitted coded signal composed of compositeparts according to claim 12 further comprising the step of: determiningthe instantaneous relative separation of a plurality of movingreceivers, with respect to one another and their individual relativeseparation with respect to the master station at the instant that thecoded transmitted signal is received at each of said receiver thatreceived said coded signal.
 16. The method for determining the time ofarrival of a discretely transmitted coded signal composed of compositeparts according to claim 12 further comprising the steps of:communicating with said three or more receivers one at each one of saidcommunication station in order to provoke the transmission of thecalculated time of arrival and the relative position of each of saidmoving receivers, provoking the transmission of the calculated time ofarrival and the relative position of each of said receivers at eachcommunication station by said receivers, calculating its own positionlocally.
 17. The method for determining the time of arrival of adiscretely transmitted coded signal composed of composite partsaccording to claim 16 further comprising the step of: transmitting theposition of said moveable unit to the master station via thecommunication stations on request by said master station.
 18. Thenetwork of communication stations according to claim 1, wherein theaccurate timing means is a global positioning system.
 19. Acommunication station comprising a receiver and a transmitter arrangedto communicate with a master station to determine the position of aremote moveable transmitting unit, said communication station furthercomprising, a signal processing system for determining the time ofarrival of a discretely transmitted coded signal from within a pagedarea, composed of composite parts, said signal processing systemcomprising: a ramp-detection means for tracing a frequency-modulatedbandwidth-limited ramp further comprising; an antenna (20) which passesfrequency-modulated signals to an envelope detector (21) for detecting abandwidth-limited ramp, increasing in amplitude up to the maximumoperational amplitude which outputs signals to a differentiator (22)which measures the rate of change in the amplitude of the ramp signaland the output of which is passed to a threshold detector (23) designedto identify a particular threshold condition such that when thiscondition is identified, the threshold detector (23) then passes asignal to a trigger circuit (24) which then reads the instantaneousvalue of a timing means (11) and passes said instantaneous value to asignal processing circuit (40), and a timing-synchronisationsignal-processing means comprising an accurate timing means to controlthe receiver's own internal timing means (11) thereby providing thereceiver with the ability to maintain a highly accurate timing means(11) at a high timing resolution, which outputs an instantaneous clockvalue to said trigger circuit (24) for transmission to the masterstation such that the master station can determine the difference intime between the master station's own timing means and the timing meansof said communication station.